Asymmetric Fault Currents

Asymmetric fault currents occurring in large power systems, or when short circuit testing a generator or motor, typically comprise a slow exponential decay DC component with a time constant of between 10 and 100ms, with a 50/60Hz waveform superimposed. 

Rogowski probes are AC current probes but the CWT LF range has an extended low frequency bandwidth, significantly less than 0.1Hz for the majority of the current range. This extended low frequency performance limits the distortion when measuring long duration pulses such as the dc component of current in these fault current applications.

Due to the high tolerance to which the LF (-3dB) performance is specified in the CWT LF the low frequency limitation, termed droop offset, is predictable and can even be compensated when post processing results.